This invention relates generally to multi-electron beam color cathode ray tubes (CRTs) and is particularly directed to an inline electron gun arrangement for correcting for spherical aberration in a color CRT.
Most color CRTs employ an inline electron gun arrangement for directing a plurality of electron beams on the phosphorescing inner screen of a glass faceplate. The inline electron gun approach offers various advantages over earlier "delta" electron gun arrangements particularly in simplifying the electron beam positioning control system and essentially eliminating the tendency of the convergence to drift. However, inline color CRTs employ a self-converging deflection yoke which applies a nonuniform magnetic field to the electron beams, resulting in an undesirable astigmatism in and defocusing of the deflected electron beam spot displayed on the CRT's faceplate. Various solutions offering a range of degrees of success have been proposed for minimizing or eliminating these two performance limitations of inline CRTs.
One of the disadvantages of the inline electron gun is the reduced lens diameter compared to a "delta" gun which causes inferior beam spot size. The ability of the electron gun to form small, symmetrical beam spots is a major factor in achieving optimum video image resolution. Incorporating this capability in present CRTs has become an even greater challenge because of reduction in the diameter in the CRT neck brought about primarily by space constraints. Reducing CRT neck diameter limits the size of the individual electron lens elements and their ability to form small, sharply defined spot images on the CRT's faceplate. The solution to the problem of providing a high resolution video image is thus complicated not only by the limited ability of the electron gun to form three separate, very small and precisely defined beam spot images, but also by the requirement to mutually converge the three spot images with the variation in the spread of the beam spots with their position on the CRT faceplate.
Recent developments in CRT design have lead to an electron gun employing an open main lens for reducing spherical aberration while improving electron beam spot size. A CRT incorporating this type of electron gun is referred to as a COTY (for combined optimum tube and yoke) CRT and employs a bipotential main lens having a pair of complementary electrodes, or grids, each having a respective common lens arranged in adjacent, facing relation. The common lens portions of adjacent lens elements provide a substantial increase in horizontal lens aperture size for reducing horizontal spherical aberration. Although also affording improvements in beam focusing and astigmatism correction, COTY-type CRTs suffer from various limitations. For example, its center electron gun exhibits greater spherical aberration in the vertical direction than the two outer guns. This appears as a larger green vertical spot size than that of the red and blue electron guns and limits video image resolution and degrades color purity. The COTY-type CRT incorporates a G3 grid having an elongated, "racetrack" shaped aperture and a G4 grid having a "dog bone" shaped elongated aperture enlarged at both ends in the common lens portions of these grids through which all three electron beams are directed.
The present invention overcomes the aforementioned limitations of prior art inline electron guns, by reducing the vertical spot size of the green color producing electron beam so as to reduce vertical spherical aberration and improve video image quality without sacrificing other performance parameters in an inline color CRT.